1. Technical Field
The invention arrangements relate to the field of LCOS (liquid crystal on silicon) and/or LCD (liquid crystal display) video projection systems. More particularly, the inventive arrangements taught herein are related to automatically adjusting the common-mode electrode voltage in LCOS/LCD projection systems.
2. Description of Related Art
In LCOS systems, it is necessary to set the common mode electrode voltage to be precisely between the positive and negative drive voltages to the pixel. It is typical to drive the imager of an LCOS display with a frame-doubled signal to avoid 30 Hz flicker, by sending first a normal frame in which the voltage at the electrodes associated with each cell is positive with respect to the voltage at the common electrode (positive picture) and then an inverted frame in which the voltage at the electrodes associated with each cell is negative with respect to the voltage at the common electrode (negative picture) in response to a given input picture. The common mode electrode voltage is denoted VITO, wherein the letters ITO denote indium tin oxide, namely the voltage at the electrode substrate of the LCOS wafer made from these materials. Setting VITO in this manner avoids both flicker and image retention, both of which can adversely affect the device lifetime. As this setting is now accomplished by an open-loop control, there is opportunity for error in VITO, and drift with time and temperature.
The typical implementation of the prior art is to use an open-loop DAC (digital to analog converter) to allow the adjustment of VITO using a fast photodiode pick-up and a visual alignment using an oscilloscope and an operator.
The present state of the art in LCOS requires the adjustment of the common-mode electrode voltage to match the positive and negative field drive for the LCOS. The balance is necessary in order to minimize flicker, as well as to prevent the phenomenon known as “image sticking”. In order to avoid visible flicker, it is common practice to use a higher frame rate, typically 120 Hz, to suppress flicker. However, the higher frame rate makes adjustment of the common mode electrode voltage more difficult, as the flicker is not visible to the human eye. An operator can not make the necessary adjustments. This can be overcome using a photodiode, or other fast detector, and balancing the AC component of the output. Unfortunately, this open-loop adjustment can be insufficient due to thermal effects in the system.
Thus, a need exists for controlling common mode electrode voltage in a LCOS/LCD in a manner that automatically accounts for the thermal effects in the system and overcomes the inability to make manual adjustments due to the higher frame rates.